The present invention relates to the processing of solid particles in fluidized beds. Its object is more specifically a method and a device to process solid particles in a fluidized bed by a fluid circulating against their flow direction, in particular in order to eliminate the components carried along with these particles and/or absorbed on them. This processing is commonly designated by the term xe2x80x9cstrippingxe2x80x9d.
The invention applies more specifically to techniques used in the oil industry, namely hydrocarbon conversion processes such as the process of catalytic cracking in fluidized bed (in English, Fluid Catalytic Cracking or FCC process).
In this type of process, the hydrocarbon charge is simultaneously vaporized and placed in contact at high temperature with a cracking catalyzer that is kept suspended in the charge vapors. After the desired mol weight range has been attained by cracking, and with a corresponding lowering of the boiling points, the catalyzer is separated from the products obtained. The catalyzer is quickly deactivated during the brief period when it is in contact with the charge, essentially due to an absorption of hydrocarbons as well as a depositing of coke and other contaminants on its active sites. It is thus necessary to continuously strip the deactivated catalyzer particles (or grains), for example by a fluid such as vapor, in order to recover from them the absorbed hydrocarbons carried along in the empty volume separating the grains, and to then regenerate them, likewise continuously, by a controlled combustion of the coke in a regenerating section before recycling the catalyzer grains to the reaction zone. These two processing operations, stripping and regeneration, are carried out in a fluidized bed.
In its type form, the stripper of the FCC process comprises only a single stirred extraction stage and thus can only be quite limited in efficacy.
The main purpose of the stripping of the catalyzer used in this process is to reduce the quantity of hydrocarbons returned to the regenerator. These hydrocarbons are divided into three categories:
interstitial hydrocarbons (between the grains),
intra-granular hydrocarbons (in the grain pore spaces),
absorbed hydrocarbons (at the grain pore surface).
Based on these three categories, three stages can be defined in stripping:
washing (moving the interstitial hydrocarbons),
dissemination, and
desorption.
It should be noted that these three operations are not independent of each other. In fact, it is clear that an efficacious washing will result in a concentration gradient between the outside and the inside of the grain. This will only improve the dissemination stage. It can be deduced from this that stripping efficacy is closely connected to desorption quality. Nevertheless, none of the three components can be neglected, since any hydrocarbon desorbing goes through the intra-granular stage and then the interstitial stage.
Use of the large industrial fluidized beds that are found particularly, but not exclusively, in these solid particle fluidized bed stripping and regeneration operations presents, however, a certain number of difficulties. The efficacy is considerably influenced on the one hand by an excessive fluidized bed concentration along the stripper axis and, on the other hand, by poor material transfer from the hydrocarbon-rich emulsion phase to the bubble phase, the only gaseous phase removed from the stripper.
It is thus necessary to carry out a consistent mixing there in order to ensure intimate mixing.
The inevitable rising of the vapor bubbles to the bed surface certainly does not allow an ideal movement of the particles against the flow. Indeed, the vapor bubbles cause catalyzer stripped from the bottom of the stripper to rise in their wake and this catalyzer reabsorbs the hydrocarbon vapors present at the surface or tossed back by grains of already stripped catalyzer. It is this phenomenon, referred to as retro-mixing, that it seems crucial to reduce in order to avoid reabsorbing hydrocarbons desorbed by already stripped catalyzer.
In addition, due to the necessary stirring of the fluidized bed, there is a sizeable quantity of non-stripped catalyzer grains that quickly go from the fluidized bed surface to the exit of the stripper. This is what is referred to as the xe2x80x9cbypassxe2x80x9d or xe2x80x9cshort-circuitxe2x80x9d, and it is important to reduce it in order to optimize the hydrocarbon extraction.
Various solutions have already been proposed.
The interposing of obstacles in the catalyzer""s dense fluidized phase can be mentioned in particular; these are in the form of insides with quite diverse structures, as described in particular in French Patent No. 2,728,805 in the Applicant""s name, or in the form of baffles. The function of these obstacles is to reduce in size or burst the vapor bubbles and thereby increase the surface of transfer toward the gas-solid emulsion while limiting the rising of the catalyzer.
This solution is limited in efficacy, however. Indeed, in practical experience, a fluidized bed is not exactly a perfect mixer. Numerous analyses have shown that although mixing along the stripper axis is quite efficacious there, radial mixing is far from satisfactory.
Consequently, in the case of homogenous vapor distribution, a stripper with internals may actually prove less efficacious than an empty stripper if these internals promote radial mixing and do not decrease axial mixing. With such internals, the conditions of a perfectly stirred reactor are better approximated and the xe2x80x9cbypassxe2x80x9d phenomenon is increased.
As another solution, PCC unit strippers are known that are equipped with multiple stage vapor injections, like the device described in U.S. Pat. No. 5,601,787. The hot-stripping housing comprises two stages arranged in the housing of the regenerator, with the stripping vapor of the second stage going through the first stage without touching the catalyzer coming from the first stage. However, this device only seems to be able to operate at high temperature.
To improve the stripping phase, we are clearly faced with a gas/solid extraction problem. The Applicant has determined, quite surprisingly, that a much surer way of improving stripping efficacy is by multiple stage extraction along several stripping chambers defined by partitions in the stripper housing in such a way that the bubbles that rise in one of the chambers are unable to carry catalyzer particles back into the (higher) preceding chamber. A multiple stage stripping with at least two chambers is thus created, with fresh stripping fluid, such as vapor, being fed into each chamber.
For this purpose, the primary object of the present invention is a method for stripping hydrocarbon-impregnated solid particles in a fluidized bed by means of a fluid circulating against the flow direction of these particles, implemented in a housing comprising, in its upper section, a diluted fluidized zone where the particles to be stripped arrive, and in its lower section, a dense fluidized bed zone, the latter comprising at least two chambers arranged basically adjacent, each of these chambers having separate means for inserting solid particles, and in its lower section, separate means for inserting gaseous stripping fluids, this method being characterized
in that the solid particles arriving from this diluted fluidized zone are directed by a directing means toward the entrance of the first chamber without being able to directly enter the second chamber;
in that the solid particles to be stripped enter the first chamber in their entirety first of all by way of the upper section of the dense fluidized bed, in which they undergo a first stripping;
in that the solid particles having undergone this first stripping are then transferred into at least a second chamber which has in its upper section gas/solid separating means allowing the gaseous fluids from the stripping of the second chamber to pass directly from the bottom to the top, into the diluted fluidized zone located in the upper section of the housing;
and in that the solid particles having undergone a second stripping in this second chamber are then removed by way of the lower section of this second chamber.
The volume mass of the dense fluidized bed contained in each of the at least two chambers preferably ranges from 400 to 800 kg/m3.
In particular, the gaseous stripping fluid is vapor or nitrogen.
The flow rate of the stripping fluid of the first chamber advantageously ranges from 1.5 to 4 times that of the subsequent chamber or chambers. This configuration allows most of the hydrocarbons carried out along by the catalyzer (and not absorbed) to be removed in the first stage of the stripper.
According to a preferred form of construction, a substantial reduction of the stripping fluid surface speed takes place in each chamber, causing smaller bubbles, to form and thus increasing the transfer of the hydrocarbons in the gaseous phase.
A second object of the invention relates to a device for stripping hydrocarbon-impregnated solid particles in a fluidized bed by means of a gaseous fluid circulating against the flow direction of these particles, comprising:
a housing provided with an upper section able to allow the forming of a diluted fluidized zone for inserting the solid particles to be stripped, and a lower section able to allow the forming of a dense fluidized stripping zone divided into at least two chambers arranged roughly adjacent and each comprising a separate inserting device for solid particles and gaseous fluid,
a pipe connected to the housing base for removal of the stripped particles, this device being characterized in that it comprises in the upper section of the second chamber at least one wall constituting at least one partition between these chambers working together with a deflector in order to remove the gaseous stripping fluids coming from the second chamber directly toward the diluted fluidized zone and to direct the fall of the solid particles to be stripped toward the entrance of the first stripping chamber.
This partition is advantageously formed in the lower section of this first chamber so as to have at least one opening for the particles to go from this chamber to the second chamber. In particular, the partition or partitions is/are arranged basically vertically.
According to a first variant, the partition or partitions share symmetric relative to the longitudinal axis of the stripping housing.
According to a second variant, the partition or partitions is/are arranged in the form of crosswise partitions.
The partitions are preferably offset from each other relative to the longitudinal axis of the stripping housing.
The deflector is advantageously arranged along the circumference of the stripping housing above the upper level of the dense fluidized bed and contiguous to the internal partition of the housing.
According to a variant method of execution, the deflector consists of an inclined partition starting from the internal partition of the housing toward its axis.
This deflector concentrates the gaseous fluids extracted from the at least two chambers above the upper level of the dense fluidized bed so as to carry out a preliminary stripping of the solid particles entering the first chamber and to limit hydrocarbon resorption by the stripped particles located at the surface of the dense fluidized bed.